One of the plague cauldrons is located in the field of a former farmhouse now referred to as Dalson's Tears. Well, we shall shed no tears this day as we thwart the Scourge and have them choke on their own fumes!

Go to Dalson's Tears, almost due north of Andorhal, and eliminate the cauldron lord there. It should have a key like the last one did, allowing you into the cauldron. Use this bottle to obtain a sample from inside the cauldron; each cauldron has its own distinct brew of toxins it uses.

The macabre key found on the cauldron lord easily slides into a keyhole and then disappears, allowing you permanent access to it. Crude, viscous ichors pulsate through twisted tubes within the cauldrons underpinnings.